1. Technical Field
The present invention relates to silicon-substrate etching methods and etching apparatuses structured to execute, repeatedly in alternation, an etching operation in which an etching gas is converted into plasma to etch the silicon substrate, and a protective-film deposition operation in which a protective-film deposition gas is converted into plasma to form a protective film on the silicon substrate.
2. Description of the Related Art
Examples of the above-mentioned etching technique known to date include the method disclosed in Japanese Unexamined Pat. App. (based on Int'l. app.) Pub. No. H07-503815. This etching technique is one in which silicon substrates are etched by placing a silicon substrate on a platform in a processing chamber and then executing repeatedly in alternation, as indicated in FIG. 8: an etching operation E, in which etching gas is supplied into the processing chamber at a constant flow rate and converted into plasma, and at the same time a bias potential is applied to the platform; and a protective-film deposition operation D, in which a protective-film deposition gas is supplied into the processing chamber at a constant flow rate and converted into plasma to form a protective film on the silicon substrate.
In the etching operation E, the etching gas is converted to plasma, generating ions, electrons, radicals, and so on. The silicon substrate is etched by the radicals chemically reacting with the silicon atoms and, as a consequence of the potential difference (bias potential) produced between the platform and the plasma, by the ions traveling toward and colliding with the silicon substrate (platform). Accordingly, in the silicon substrate, which is covered with a photoresist mask of a predetermined pattern (lines, circles, etc.), those areas not covered by the mask are etched, forming grooves and holes provided with predetermined width and depth.
Meanwhile, in the protective-film deposition operation D, the protective-film deposition gas is converted to plasma, and, as is the case with the etching gas, ions, electrons, and radicals are generated. The radicals create polymers, and the polymer formation is deposited on the sidewalls and bottoms of the grooves and holes, whereby a protective film that does not react with the radicals generated by the etching gas is formed.
Thus, according to the etching technique of repeating the etching operation E and the protective-film deposition operation D in alternation: in the etching operation E, along the bottoms of the grooves or holes, where the ion bombardment is heavy, protective film removal by ion bombardment and etching by radical and ion bombardment proceed, and along the sidewalls of the grooves and holes, where the ion bombardment is slight, only protective film removal by ion bombardment proceeds, with etching of the sidewalls being prevented; and in the protective film deposition operation D, polymers are deposited on the bottoms and sidewalls again to form a protective film. Accordingly, the new groove and hole sidewalls formed by the etching operation E are immediately protected by the protective film formed by the protective-film deposition operation D, whereby etching progresses only along the depth of the grooves and holes.
It should be noted that the gases inside the processing chamber are exhausted to the exterior at a constant flow rate by means of a suitable exhaust device; this device reduces the pressure inside the processing chamber, and at the same time discharges outside the processing chamber the etching gas and the protective-film deposition gas consumed in etching the silicon substrate and forming the protective film.
In order to execute the protective-film deposition operation D after execution of the etching operation E, it is necessary to supply the protective-film deposition gas into the processing chamber, to exchange or replace the etching gas in the processing chamber with the protective film deposition gas. Likewise, in order to execute the etching operation E after execution of the protective-film deposition operation D, it is necessary to supply the etching gas into the processing chamber, to exchange or replace the protective film deposition gas in the processing chamber with the etching gas. However, this exchanging of gases requires a certain amount of time.
Consequently, for a certain time following the transition from the etching operation E to the protective-film deposition operation D and following the transition from the protective-film deposition operation D to the etching operation E, the etching gas and the protective-film deposition gas become mixed, and the etching process induced by the etching gas and the protective-film deposition process induced by the protective-film deposition gas proceed simultaneously. As a result, the etching of, and the forming of the protective film on, the silicon substrate that should be carried out by the etching operation E and the protective-film deposition operation D cannot be adequately performed.
Accordingly, several problems occur in situations in which it takes a long time to replace the gases: the etch rate is lowered; high-precision etching profiles cannot be obtained because a high-quality protective film does not form, leading to inadequate protection of the sidewalls; and mask selectivity is lowered because the protective effectiveness of the mask is weakened. These problems are especially pronounced when processing times in the etching operation E and protective-film deposition operation D are short, such that the number of gas replacements is large during a given etching process time, or when the etching operation E and protective-film deposition operation D are performed under high pressure, wherein it takes time to exchange the gases.
However, in the above-described conventional etching technique, the gases are exchanged by exhausting the gas within the processing chamber at a constant flow rate and at the same time supplying the etching gas or protective-film deposition gas into the processing chamber at a constant flow rate, wherein a problem with the technique has been that time required to exchange the gases is prolonged, as will be understood from the fact that, as indicated in FIG. 9, after the gas supply is halted it takes a long time for the pressure in the processing chamber to stabilize (until the gas in the processing chamber is completely exhausted) and from the fact that, as indicated in FIG. 10, after the gas supply is started it takes a long time for the pressure in the processing chamber to stabilize (until the gas completely fills the processing chamber). Herein, FIG. 9(a) and FIG. 10(a) are graphs plotting the relationship between gas-supply flow rate and time in a conventional example, while FIG. 9(b) and FIG. 10(b) are graphs plotting the relationship between pressure within the processing chamber and time in the conventional example.
Furthermore, as a consequence of a time lag, as indicated in FIG. 9(a) and FIG. 10(a), that occurs between the controlled supply flow rate that is the control target value and the actual supply flow rate, system disturbances arise, such as the continuance of the etching-gas or protective-film-forming-gas supply even after the close of either operation (after transitioning from one operation to the other), and the occurrence of a time interval during which gas supply into the processing chamber immediately after the start of either operation does not take place. These factors also rule out the efficient exchange of gases within the processing chamber.
Consequently, the conventional etching technique discussed above has not allowed the etch rate to be quickened or the mask selectivity to be heightened, nor has it allowed high-precision etching profiles to be obtained.